Police are investigating whether recent violence in Chiang Mai, Bangkok and Chon Buri is related to the political conflict.
Police have stepped up security for certain independent agencies and courts, as well as at the homes of certain important figures and judges following recent violent attacks, according to Royal Thai Police deputy spokesman Pol Maj-General Anucha Romayanant.
Grenades were fired and thrown at locations linked to agencies and officials dealing with cases against government politicians.
A police meeting yesterday, chaired by deputy national police chief Pol General Aek Angsananont, agreed to the need to heighten patrols around those locations in order to prevent more attacks by "people with ill intentions who want to create some incident", the spokesman said.
The focus would be on the risky areas within a 400-metre radius around those places, he said, adding that police checkpoints have been set up in some areas.
Police also would ensure that security cameras and bomb-disposal personnel would be ready for action in case they are needed.
A distance of 400 metres is within the effective range of grenades fired from an M79 launcher, which has become the weapon of choice for the perpetrators in attacks over recent years.
Three separate violent incidents involving grenades occurred in the northern city of Chiang Mai on Friday night, leaving four people injured - one of them seriously, according to police.
In the eastern seaboard province of Chon Buri, two grenades were fired into a building that houses the Bang Sai administrative organisation's office at about 9.30pm on Friday. Part of the building and two cars parked inside were damaged. There were no reports of injuries.
Police said the intended target of the attack could be the nearby rally site of the anti-government protesters linked to the People's Democratic Reform Committee (PDRC). The rally had attracted about 2,000 people when the attack occurred.
PDRC local leader Kantapon Sukhumalin said it was lucky that the grenades fell short of the rally site, otherwise there could have been many injuries.
Separately, two home-made bombs planted in a car exploded early yesterday in front of a house in Lak Si in Bangkok. The car had been parked there for four days.
The explosions, which set the car ablaze, took place at about 5am outside the house of a senior Royal Thai Air Force officer, who police said had no involvement with the ongoing political conflict.
Pol Colonel Kamthorn Uicharoen, from the police's Explosive Ordnance Disposal (EOD) unit, said the bombs were put in cooking-gas cylinders and the way they were assembled was similar to one found recently outside the Attorney-General's Office.
In Chiang Mai, a string of grenade blasts on Friday night came just hours after the Constitutional Court nullified the February 2 general election - a ruling that angered government supporters.
The targets of attacks were a seafood restaurant, a petrol station and a brewery that has faced criticism because of its members of the owner's family have links with anti-government protesters.
Four people were wounded but were out of danger yesterday and had returned home, Chiang Mai provincial police commander Grit Gitilue said. "We established two possible motives for the attacks - personal conflict or politics. We are giving more weight to the second one," he said, adding that no suspects had been arrested.